Optical fibers are conventionally made from glass rods or preforms having a central core of glass enveloped by a cladding of glass whose refractive index is lower than that of the glass core. The fiber is produced by heating the glass preform to softening temperature in a furnace and drawing the fiber from the softened preform. The fiber is rapidly cooled sufficiently to enable a protective coating of resin material to be applied to the surface of the drawn fiber. The cooling is carried out by drawing the fiber through a heat exchanger wherein it comes into contact with a gaseous coolant.
The gaseous coolant is continuously passed through the heat exchanger in a direction which is cross-flow, counter-flow, co-current-flow, or combination mode thereof relative to the direction of the movement of the glass fiber through the heat exchanger. The gaseous coolant transfers heat from the glass fiber to the walls of the heat exchanger which is cooled by the surrounding atmosphere and/or a cooling medium, usually water, which flows through passages in the heat exchanger. The gaseous coolant is generally helium although other gases or mixtures can be used. Helium is the preferred coolant gas because of its favorable heat transport properties and is safe to use. However, helium is costly relative to other gases so it is desirable to capture the helium and recycle it for reuse in the heat exchanger.
If the exhausted helium gas from the heat exchanger is vented to the atmosphere as what has been done in the current fiber production processes, the fiber production cost would be disadvantageously higher especially as the draw speed of fiber has kept increasing over the last 30 years. In order to reduce the fiber cost associated with helium use in the fiber cooling step, helium recovery systems and apparatuses have been proposed to recover the helium gas. Exhausted helium gas with contaminants such as air and moisture is evacuated from the heat exchanger and purified before being is recycled back to the coolant gas feed stream to the heat exchanger.
However, these helium recovery systems suffer from one or more of the following drawbacks: (1) fiber vibration as a result of the ambient air flow at the fiber inlet or outlet where the helium is collected and vacuum is applied; (2) fluctuation in fiber diameters due to lack of control in the pressure, composition, and flow rate of the recovery system, which would negatively affects the fiber's mechanical and optical quality; (3) lower coolant (such as helium) purity and recovery due the ingress of air into the collected stream and egress of coolant (such as helium) from heat exchanger.
Air infiltration can be reduced significantly by ensuring there is a positive differential pressure between the cooling gas inside the heat exchanger and the surrounding environment. This has a disadvantage in that valuable helium will be lost to the environment through the fiber inlet end opening and/or fiber outlet end opening of the heat exchanger. Efforts have been made to minimize the amount of helium efflux and air influx through the fiber inlet end and outlet openings. For example, controlling the flow of helium into and out of the heat exchangers to limit air infiltration into the heat exchangers. However, operating heat exchangers at atmospheric or superatmospheric pressure results in significant loss of coolant gas from the system. A more economical processes for producing optical glass fiber are constantly sought.
The present invention provides a novel cap assembly for collecting the cooling gas with high purity and high recovery efficiency to reduce the cost of producing optical glass fiber without causing any negative impact on the fiber production process.